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Automatic dampers control airflow in HVAC systems. Applications include controlling mixed air temperature in economizer cycles and airflow to the space in variable air volume systems. Selecting and sizing dampers to provide linear control action aids proper system operation. With linear control, a given change in damper position produces a proportional change in air quantity. If control is not linear, a given change in control signal might produce a consistent change in damper position but a different change in air quantity. The result is unstable or inaccurate control.
The resistance of the wide open damper can be expressed as a fraction of the total system resistance. That fraction is called "Damper Authority" or "Characteristic Ratio."
Note: Total system resistance and total system pressure drop are for the system without the wide open damper.
It is important to realize that "Total System Resistance" or "Total System Pressure Drop" relates only to the part of the system where the damper controls the flow. It is not the entire system pressure drop or the fan total static pressure. The total system pressure drop for selecting dampers is usually the pressure drop from a constant pressure point in the system to the destination for the air.
Figure 1 shows a traditional airside economizer cycle with a return fan.
The total system pressure drop for the outside air damper is the pressure difference between outside air and Point A, the mixing box plenum. The outside air damper only controls the flow through the weather louver, the damper, and the outside air ductwork. Therefore, the outside air damper authority is determined by the pressure drop across the outside air damper as a percentage of the pressure difference between outside air and Point A, the mixing box plenum. The outside air damper only controls the amount of flow. It does not control the direction of flow. For outside air to enter the system, other features of the control system must make the pressure in the mixing box (Point A) less than atmospheric.
Similarly, the total system pressure drop for selecting the exhaust/relief damper is the pressure drop between Point B and outdoors. For determining the damper authority, the total system pressure drop includes the pressure drop across the relief duct and louver....